Micro Displacement how to make it to look better, or is there a better solution (5 questions)
Lets go from big issues to small ones. 

1.The rocky, jagged surface you're experiencing is a the fact that your displacement is too strong. Add a Math node, set it to multiply and put it between the displacement texture and the displacement output in the node editor. You can then adjust the second value to decrease the height, to make the displacement more subtle and thus convincing. 

2.The original plane geometry doesn't go away, it just gets subdivided more when you hit render. What you're encountering is the preview mode - in a render preview, the mesh is subdivided less to improve performance. That is the "Dicing scale" option in the subdivision surface modifier. The smaller it is the more detailed the mesh will be - both for rendering and preview. Decrease the dicing scale if the issue persists during rendering. 

3.The thing with textures is that they always add surface detail. By that I mean that certain surface imperfections are just in the Albedo, but not in the displacement. Those will dissappear if you use a different texture.  If you go too detailed with your displacement texture you'll eventually encounter the limits of whatever image format you're using. The basic displacement can stay the same, and you can slap a different texture on top without issues. Keep in mind tho that the displacement and Albedo textures tend to go hand in hand tho. Out of curiosity, why did you blur the displacement texture?

4.This feature is coming to blender, but not there yet. You can refresh the mesh by tabbing into edit mode and out again when you've made changes. 

5.You can definitly sculpt such detailed wall ornaments, although sculpting is quite different. It will certainly take more time to sculpt it, a micropolygondisplacement will definitly be faster to implement. Depending on your final sculpting implementation it might be more ressource intensive as well.